Building Energy Efficiency

The envelope loses and gains heat four ways, and fabric-first design pulls four matching levers. Insulation slows conductive heat flow through walls, roof and floor; its performance is measured as a U-value (watts lost per square metre per degree - lower is better). A poor solid wall might be U 1.5-2.0; a decent modern wall 0.25-0.30; a Passivhaus wall 0.10-0.15. Roofs, which lose the most in cold climates and gain the most under a hot sun, are pushed lowest of all. Airtightness stops uncontrolled draughts, which in a leaky building can account for a third or more of heat loss; it is measured in air changes per hour at 50 pascals (ACH50) - ordinary construction manages perhaps 5-10, good practice 3, and Passivhaus demands 0.6 or below. Crucially, airtight does not mean unventilated: you seal the fabric and ventilate deliberately, usually with heat recovery.

Glazing is the weakest part of the envelope and the biggest lever for both heat and light. Single glazing is around U 5.0; standard double 1.4-2.8; argon-filled, low-emissivity triple glazing 0.8 or less. But glazing is two-sided: in a hot climate you also manage solar heat gain (the g-value or SHGC) and orientation, or a well-insulated window becomes a greenhouse. Thermal bridging is the fourth, most-missed lever: heat shortcuts through balconies, slab edges, lintels and junctions where insulation is broken. Unaddressed, thermal bridges can add 10-30% to heat loss and cause condensation and mould. Fabric first means designing continuous insulation, an unbroken airtight line, appropriate glazing, and detailed junctions - together, before any plant is sized.

Once the fabric has shrunk the load, efficient systems meet what remains. HVAC is usually the largest single end-use - often 40-50% of a building's energy - so it is where efficient plant pays back hardest. Heat pumps deliver three to four units of heat or cooling per unit of electricity (a seasonal COP of 3-4), against a gas boiler's less-than-one; variable-speed fans and pumps, correctly sized ducts and pipes, and mechanical ventilation with heat recovery (MVHR) that reclaims 75-90% of the warmth in exhaust air all cut waste. Oversized plant is a hidden efficiency killer: a chiller or boiler far bigger than the leaned load cycles inefficiently and costs more to buy and run.

Lighting has been transformed by LEDs, which use 75-85% less energy than incandescent and last far longer; paired with daylight-linked dimming and presence detection, lighting energy can fall by 60-80% against older installations. Appliances and plug loads - increasingly a large share as fabric and HVAC improve - are cut by specifying high-rated equipment (BEE star ratings in India, ENERGY STAR elsewhere) and by simply switching off standby loads. The rule across all systems is the same: right-size to the leaned demand, choose high-efficiency equipment, and never let a machine run when it need not.

Efficient fabric and efficient plant still waste energy if they run at the wrong times, and this is where controls earn their keep. Good controls are often the cheapest efficiency measure of all: time schedules that match occupancy, thermostats and setpoints that are not fighting each other, zoning so empty rooms are not conditioned, CO2-based demand-controlled ventilation, daylight dimming, and a building management system that a competent operator actually understands. Setpoint discipline alone matters more than people expect - widening a comfort band by even one or two degrees, or letting temperatures drift when a space is empty, saves real energy at no capital cost.

But the honest heart of efficiency is measurement, because designed performance and measured performance routinely diverge - the performance gap. Real buildings often use 1.5 to 2.5 times the energy their design models predicted, because of poor construction quality (insulation gaps, failed airtightness), controls left in override, and occupants behaving unlike the model. This is why London's hierarchy adds be seen: monitor the building in use, sub-meter major end-uses, and feed what you learn back. There is also a rebound effect to respect: efficiency that makes energy cheaper can lead people to use more of it, eroding part of the saving. None of this argues against efficiency - it argues for verifying it, commissioning properly, and designing for how buildings are actually run, not how a spreadsheet assumes they are.

It is tempting to treat efficiency as a new-build subject, but the larger prize is the stock already standing - most of which will still be in use in 2050, and most of which is leaky, uninsulated and running old plant. Fabric-first logic applies to retrofit just as much as to new work, but the levers are harder to reach: you cannot re-orient an existing building or easily rebuild its walls, so retrofit efficiency is a craft of the possible. The usual order is to start with the cheap, high-impact moves - air-sealing (draught-proofing, sealing service penetrations and loft hatches), adding loft or roof insulation, and upgrading controls and lighting - because these deliver large savings per rupee or dollar with little disruption. Then come the deeper, costlier measures: external or internal wall insulation, window upgrades, and replacing fossil plant with a heat pump, ideally staged so the fabric is improved before the new plant is sized, or the heat pump ends up oversized for a load that later shrinks.

Retrofit carries risks that new-build does not, and honest efficiency respects them. Insulating and air-sealing an old building changes how it handles moisture; done without attention to ventilation and vapour movement, it can trap damp and cause condensation, mould and even fabric decay - so ventilation (often MVHR or at least controlled extract) must be designed in alongside the sealing, never as an afterthought. Heritage and solid-wall buildings need particular care. The reward for getting it right is large: a deep energy retrofit can cut an existing building's energy use by 50-80%, and doing it reuses the embodied carbon already locked into the structure rather than spending fresh carbon on demolition and rebuild. This is a whole subject in its own right - Module 9 develops deep retrofit, adaptive reuse and the retrofit-versus-rebuild judgement in full - but the efficiency principles are the same ones this lesson has set out, applied to a building you did not get to design from scratch.

Stacked together, fabric-first design and efficient systems and controls can cut a building's energy use by 40-70% against a conventional baseline before a single renewable is added - which is exactly why the hierarchy puts them first. Passivhaus, the most rigorous fabric-first standard, caps space-heating (or cooling) demand at around 15 kWh/m2 per year and total primary energy at roughly 120 kWh/m2 per year - numbers a typical building misses by a wide margin. You do not have to reach Passivhaus to benefit; each lever moves the building meaningfully down the demand cascade.

India's codes make efficiency measurable and mainstream. The Energy Conservation Building Code (ECBC) sets envelope, lighting and system requirements for commercial buildings, and Eco Niwas Samhita does the same for homes, both often expressed through an Energy Performance Index (EPI) in kWh/m2 per year that a design must beat. In hot Indian climates the fabric priorities shift - the biggest wins are shading, cool or insulated roofs, reduced and shaded glazing, and controlling solar gain rather than chasing ultra-low winter U-values - but the principle is identical: cut the load through the box first. Efficiency is not glamorous, but it is the rung that makes net-zero affordable.

One last framing keeps efficiency in proportion: it is the only energy measure that is simultaneously the cheapest, the most permanent and the most reliable. A saved kilowatt-hour never has to be generated, transmitted, stored or paid for; it works in a blackout; and it keeps working for the life of the building regardless of who owns it or what the tariff does. That is why the whole module puts it first - and why the next lesson turns to supplying the demand that even the best fabric cannot design away.